1. Field of the Invention
The present invention relates to novel coating powders for use in the preparation of protective films. In particular, the present invention relates to solid, non-caking, electrostatically sprayable coating powders that are conveniently and easily handled, which upon heating cure to provide protective films having outstanding properties
2. Description of the Prior Art
Powder coatings have been widely used since the late 1950's. Initially, one-component polyurethane coating systems were used, which involved stable isocyanate-terminated prepolymers that were cured by means of the reaction of the free isocyanate group with water or atmospheric moisture.
Later, "blocked" isocyanates began to be used in coating systems. These early "blocked" isocyanate systems involved the reaction of polyisocyanates with mono-functional hydroxy-containing compounds ("blocking compounds") to give products that did not react with hydroxy-containing compounds at room temperature, but at elevated temperatures the hydroxy-containing "blocking" group was eliminated regenerating the isocyanate groups which were then available for cross-linking in the unusual manner. Phenols were widely used in these early systems. Generally, however, the phenol-blocking agents were found to be somewhat objectionable since they gave blocked polyisocyanates that required higher temperatures to dissociate. Although these blocked polyisocyanates are sensitive, in general, to heat, they have the advantage of not being as sensitive to moisture as other types of urethanes. Consequently, they may be exposed for considerable periods of time to atmospheric moisture without any significant danger of decomposition. They also have the advantage over the moisture-cured urethanes in being able to be formulated very near the 1:1 isocyanate:hydroxy ratio, thereby avoiding the formation of urea groups in the structure of the final polymerized coating. Additionally, they are easily handled and applied.
It has now been surprisingly found that caprolactam "blocked" isocyanate prepolymers can be used as cross-linking agents in hydroxy-containing powders that are capable of forming protective films when heated. Upon reaction of a suitable polyisocyanate with caprolactam, there is obtained a blocked isocyanate prepolymer which is solid and non-caking and can thus be easily formulated with solid hydroxy-containing acrylic, methacrylic or polyester resins.
The resulting powder coating can easily be cured by heating the powder film to about 325.degree.-400.degree.F whereupon the free isocyanate groups are regenerated and react with the hydroxy-containing polyester or acrylic or methacrylic resins incorporated in the powder coatings resulting in the formation of protective films having urethane linkages. It has been found that the present blocking system for powder coatings offers unique and outstanding advantages when compared to the use of polyisocyanates blocked with other blocking agents, such as methyl ethyl ketoxime, phenol, and alcohols, since the caprolactam-blocked polyisocyanates of the present invention have better flow and leveling of the protective film before final curing is completed. Further, it has been surprisingly found that the caprolactam-blocked organic polyisocyanates of the present invention do not "gas" during the curing process. It will be recognized by those skilled in the art that this avoids the formation of pin-holes and other objectionable surface patterns in the finished protective film, which commonly occur when conventional blocking agents are used with organic polyisocyanates.
Further, since regeneration of the free isocyanate groups from the blocked organic polyisocyanates occurs at a slower rate than with the conventional blocking agents, thicker films may be obtained.
More importantly, the instant blocking systems admirably satisfy the requirements for powder coatings in that they are solid, non-caking materials which can be easily electrostatically sprayed onto various articles and objects.
The present invention thus provides a unique composition ideally suited for use in the preparation of protective films.